ABSTRACT
In order to combat the growing problems associated with biological invasions, many researchers have focused on identifying which communities are most vulnerable to invasion by exotic species. However, once established, invasive species can significantly change the composition of the communities that they invade. The first step to disentangling the direction of causality is to discern whether a relationship with other vegetation exists at all. Carduus nutans and C. acanthoides are similar invasive thistles, which have caused substantial economic damage worldwide. We assessed the associations between the thistles and the standing flora in four sites in central Pennsylvania in which they co-occur. After sampling nearly 2000 plots of 1 m(2), we used partial Mantel tests to assess the differences in vegetation between thistle and non-thistle plots after accounting for location, and non-metric multidimensional scaling to visualize differences among plots and sites. We found significant differences in community composition in plots with and without Carduus thistles. The non-native species Sisymbrium officinale and Coronilla varia were consistently associated with the presence of Carduus thistles. Several species were associated with areas that were free of Carduus thistles, including an important non-native pasture species (Trifolium repens). We found no evidence for differences in composition between plots with C. nutans versus C. acanthoides, suggesting that they have similar associations with the vegetation community. We conclude that even at the within-field scale, areas invaded by Carduus thistles have different vegetation associations than uninvaded areas, allowing us to target future research about the role of vegetation structure in resisting and responding to invasion.
ABSTRACT
The successful establishment of invasive species has been shown to depend on aspects of the invaded community, such as gap characteristics. Biotic resistance may be particularly critical for stopping invaders at early life history stages, but new species can often invade following disturbances, which may create microsites with very different characteristics than are usually present. We examine the response of two invasive thistle species, Carduus nutans L. and C. acanthoides L., to three different microsite characteristics: disturbance type, size, and water availability. The two species initially responded differently to the type of disturbance: C. acanthoides had higher emergence and survival in plots with both above- and belowground disturbance, whereas C. nutans had better early performance in large microsites with above-ground disturbance only. Later in their life cycle, C. nutans performed better in plots that had been disturbed both above- and belowground, whereas C. acanthoides was largely unaffected by disturbance type. Increased emergence and survival, larger size and a higher proportion flowering were observed in larger gaps for both species throughout the life cycle. Watering had a negative impact on C. nutans emergence and fall survival and on C. acanthoides survival to the following summer. Overall, these results suggest that disturbance-generated microsite characteristics (disturbance type and size) may have large impacts on establishment of these two Carduus species, which in turn may persist well beyond the initial stages of growth. Studying invader responses to disturbance can help us to understand under what circumstances they are likely to establish and create persistent problems; avoiding or ameliorating such situations will have significant management benefits.
